Conductive compositions are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. For example, conductive adhesives are used to bond integrated circuit chips to substrates (die attach adhesives) or circuit assemblies to printed wire boards (surface mount conductive adhesives).
The leading technique used throughout the electronics industry for soldering components to the substrate uses a metallic solder alloy which is so called eutectic solder containing by weight 63% tin and 37% lead. It is applied to the circuit board as a paste which is heated to above its melting temperature (183° C.) to let solder paste melt and form joint. Alternatively, the board is passed over a molten wave of solder to form joints to bond the electrical components to the circuit board. In either case, a flux material is used to remove surface oxidation from metallic surfaces and allow the molten solder to strongly bond to the surfaces and form reliable solder joints with excellent impact resistance. While the solder technology has existed for many decades, it has several shortcomings. Lead in the alloy is not environmental friendly. Numerous environmental regulations have been proposed to tax, limit, or ban the use of lead in the electronic solders. Secondly, high process temperature requires to use more expensive thermo-stable substrate and does not fit flexible substrate that becomes more popular in the electronic industry. Another shortcoming is extra step to clean up the residue from flux material after reflow process which is an expensive and inefficient process.
Conductive adhesives offer several advantages over traditional solder assembly due to the absence of lead, low processing temperatures and a simplified assembly process that does not require solder flux and subsequent flux cleaning steps. Among the desired properties of conductive adhesives are long work life at room temperature, relatively low curing temperature and relatively short curing time, good rheology for screen printing, sufficient conductivity to carry an electric current when cured, acceptable adhesion to the substrate when cured, stable electrical resistance at high temperature and humidity over long periods of time, and good impact strength.
Previous testing of commercially available adhesives has concluded that conductive adhesives are suitable for only niche applications due to the limitation by resistance and impact requirements. Some vendor claimed success at develop an impact resistance adhesive and some vendor claim success at stable contact resistance. But none of them has been successful to achieve both impact resistance and stable contact resistance, especially with tin/lead surface. In fact, many adhesive vendors have acknowledged that impact strength and resistance stability are mutually exclusive parameters.
Two conductors with dissimilar electrochemical potentials will form an electrochemical cell in the presence of water. The conductors act as cathode and anode, and environmental humidity provides the necessary aqueous medium to bridge the anode and cathode. The metal with the lower electrochemical potential acts as the anode resulting in the loss of electrons {M−ne→Mn+} and the corrosion of the metal. The metal with the higher electrochemical potential acts as the cathode {2H2O+O2+4e→4OH−}. Oxygen is involved in this mechanism but does not directly react with the anode metal. The metal ion Mn+ will combine with OH− and form a metal hydroxide that stabilizes by developing into a metal oxide, which over time forms on the anode surface. Metal oxides in general are non-conductive, the result being a decrease in conductivity of the metal circuitry.
The problem is less acute when the filler in the composition is the same metal as the contiguous circuitry or the substrate. Thus, a semiconductor package using a conductive composition, one comprising an epoxy resin and silver filler, for example, will not be as susceptible to electrochemical failure when a silver-filled composition is used on a silver substrate. However, if the composition is used on a nickel-plated substrate, electrochemical corrosion will result under high humidity conditions.
These compositions, however, are vulnerable to environmental conditions, and high temperature and high humidity can cause the electrical resistance of the assembly fabricated with these compositions to increase substantially over time. The suspected mode of failure is electrochemical corrosion of the circuitry at the interface of the conductive filler in the composition with another contiguous metal surface, for example, a metal lead frame or other circuitry.
Another vulnerability of the compositions is their resistance to impact when the packaging containing the composition is dropped or struck. Most conductive adhesives are often thought as high Tg rigid materials. This rigidity will prevent effectively dissipate mechanical energy at impact test temperature, i.e. ambient temperature. Thus conductive adhesive cracks or component falls off from substrate when the whole packages drop from height. With high filled conductive filler, the impact resistance of conductive adhesives becomes even worse.
It would be an advantage, therefore, to provide conductive materials that form electrically stable assemblies for use in semiconductor packaging operations. It would also be advantageous to provide a conductive adhesive which would provide improved contact resistance when exposed to harsh environmental conditions and subjected to an impact. At the same time, it is important that the adhesive have a relatively long shelf life.